The collection efficiency of photovoltaic cells depends on many factors, including the design of the conductor system that is used to conduct currents from charge carriers of mutually opposite polarity in the cell from the emitter and base. In the art, two classes of conductor systems are known: one class in which electrically separate grids are provided for the emitter and base on the front and back surface of the cell and one class in which electrically separate grids for the emitter and base are provided on the same surface (the back surface). Conventionally grids are used that have one or more relatively wide central conductor lines, called the bus bar and parallel straight linear fingers of relatively narrow elongated shape emanating from the central conductor line. Thus effectively, H shaped connection pattern are provided, the bus bar forming the horizontal line of the H.
U.S. Pat. No. 6,573,445 shows photovoltaic cells with separate base and emitter grids on opposite surfaces wherein the density of the fingers is varied with distance from the central line. The document also shows photo-voltaic cells wherein the area covered by the front surface grid is reduced by using repeated sub-regions each with conductor lines that radiate from one or more concentric circular lines, with an increasing number of bifurcations in the radiating lines with distance from the centre.
In the class of photovoltaic cells wherein both electrically separate grids are provided on the back surface, interdigitated conductor grids are used. Such cells are called IBC cells (Interdigitated Back Contact cells). Each grid has a H- or E-shaped pattern with a series of straight parallel fingers of a long narrow shape emanating from a bus bar. The fingers of both grids run in parallel, the length of each finger of one grid extending between the lengths of a different pair of fingers of the other grid. In this way each fingers of both grids can be realized in close proximity of a finger of the other grid.
However, the bus bars give rise to efficiency loss due to enhanced recombination underneath the central lines, which is sometimes called electric shading.